The present invention relates to a highly efficient four-cavity velocity modulation tube of reduced size and especially of reduced overall axial length.
In a multicavity velocity modulation tube, the respective cavities are tuned to mutually different frequencies within the operating passband to optimize the gain versus frequency characteristics. In conventional four-cavity velocity modulation tubes, the input cavity is tuned to the center frequency of the operating passband or to a frequency higher than the center frequency, while the pre-intermediate cavity disposed downstream of and next to the input cavity is tuned to a frequency lower than the center frequency. For details of the tube of this type, reference is made to K. H. Kreuchen, B. A., Auld and N. E. Dixon, "A Study of the Broadband Frequency Response of the Multicavity Klystron Amplifier" (Journal of Electronics, Vol. 2, p.p. 529-567; May 1957) and Tatsuo Hayashi "UHF-TELEVISION SOCHIN TO JUSHIN (Transmission and Reception of UHF-Television)" (DENPA SHINBUN SHUPPANBU January 1970, p. 272: written in Japanese). Accordingly, in the proximity of the center frequency of the operating passband, the phase of the voltage induced across the gap space within the pre-intermediate cavity is kept almost out-of-phase with respect to the gap voltage within the input cavity, so that debunching effects may possibly occur in the electron beam which has undergone bunching once. Also, in the prior art the Q-value of the cavity is higher in the pre-intermediate cavity than in the input cavity. In addition, with regard to the drift tube lengths of a velocity modulation tube of a reduced overall length, the normalized length of the first drift tube disposed downstream of the input cavity is set, with the power gain in view, at about 70.degree. in terms of the reduced plasma angle because the level of the radio frequency voltage generated across the gap space within the input cavity is deemed as a small signal that is sufficiently small relative to a D.C. beam voltage (Reference is made to S. E. Webber "Ballistic Analysis of a Two-Cavity Finite Beam Klystron", IRE Trans. on Electron Devices, Vol. ED-5, p.p. 98-108: April 1958). Since the levels of the voltages respectively generated across the gap spaces within the pre-intermediate and post-intermediate cavities are sufficiently high, normalized lengths of second and third downstream drift tubes are set at about 40.degree. and 25.degree., respectively, depending upon the levels of the voltages induced across the cavity gap spaces. The above-mentioned arrangement of drift tube lengths is effective when bunchings in an electron beam are successively and cumulatively achieved. However, since debunching occurs at the gap space in the pre-intermediate cavity, a normalized length of about 40.degree. for the second drift tube is not sufficient for the once debunched electrons to be rebunched. For these reasons, conventional four-cavity velocity modulation tubes have a limited saturation output efficiency of 30 to 40%.
It is to be noted here that a normalized length of a drift tube as represented in terms of the reduced plasma angle is given by .omega.q/Uo l, where .omega.q (radian/second) represents the reduced plasma angular-frequency, Uo represents the D.C. beam velocity, and l represents the physical length of the drift tube as measured between the centers of the gap spaces.
As described in Japanese Patent Application No. 24221/71, laid open to public inspection as a Japanese Patent Disclosure corresponding to U.S. application Ser. No. 28,792 now U.S. Pat. No. 3,622,834, the conversion efficiency of multicavity velocity modulation tubes can be improved by inserting a drift tube of a normalized length of 120.degree. between the post-intermediate cavity and the pre-intermediate cavity located just upstream thereof to allow electrons lying in the area between the centers of bunches to be shifted toward the respective bunching centers by the electrostatic force attributed to the second harmonic space charge generated in the electron beam, thereby strongly bunching the electrons. However, this method, based on the electrostatic force exerted by the harmonic space charge, requires an extraordinarily long drift tube of a normalized length of 120.degree.. As a result, overall length of the tube becomes inevitably larger than conventional tubes having ordinary drift tubes whose normalized lengths are 90.degree. or less.
Therefore, such a velocity modulation tube based on the second-harmonics-space-charge-exerting force does harm rather than making a positive contribution to the manufacture of samll-sized UHF-TV broadcast transmitters.
Furthermore, the operating frequency range and D.C. beam voltage ranges of the tube satisfying the condition of 120.degree. in normalized length are extremely limited.
On the other hand, with respect to conventional velocity modulation tubes having normalized lengths of 90.degree. or less, the conversion efficiency can be increased by making the drift length large enough to eliminate debunching without resorting to the bunching effect of the second harmonics or by undertaking appropriate tuning of the respective cavities. For further details of this technique, reference is made to U.S. Pat. No. 3,819,977 and copending U.S. Pat. application Ser. No. 552,436 filed Feb. 24, 1975, now U.S. Pat. No. 3,942,066 which application is a continuation-in-part application of U.S. application Ser. No. 408,186 filed Oct. 19, 1973, now abandoned. As is fully described in these prior specifications, this technique, when applied to a five-cavity velocity modulation tube, yields a saturation conversion efficiency of as high as 60%. However, compared with the four-cavity velocity modulation tube, the five-cavity-type tube has one excessive cavity resonator, and consequently is larger in overall length, more costly to manufacture and more difficult to adjust for optimized operation.
On the other hand, the demand has recently risen for high efficiency four-cavity velocity modulation tubes of the output cavity type and having a power output of as high as 20 KW but of moderate power gain, particularly because the improvement in transistor technology has made it possible for the exciter circuits to provide sufficiently high driving signal levels. Under the circumstances, the mere application of the cited technique (U.S. patent Appln. Ser. No. 552,436 now U.S. Pat. No. 3,942,066 referred to above) to the four-cavity velocity modulation tube does not lead to the result desired. More specifically, even though the conversion efficiency might be improved to a level of 40% to 55% by reducing the lengths of the drift tubes in proportion to the radio frequency voltages induced across the cavity gap spaces, a conversion efficiency of 60% or more, which can be realized with the five-cavity tubes, is very difficult to attain without appreciably affecting (i.e., reducing) the operational passband.